Quick tip: Type of tokio spawn return

When I was implementing the metrics task using tokio, I wanted to save the result JoinHandle in a struct and I saw the type being displayed by the IDE: JoinHandle<?>

What does it mean?

When I looked the definition of the function spawn, that’s the code:

 pub fn spawn<F>(future: F) -> JoinHandle<F::Output>
 where
 F: Future + Send + ‘static,
 F::Output: Send + ‘static,
 {
 // …

In the Future trait, F::Output refers to the return value of the input function.

Now, the piece of code I have is a long running task. That is the piece of code.

 let receiver_task = tokio::spawn(async move {
 println!(“Starting metrics receiver”);
 while let Some(event) = rx.recv().await {
 if let Err(e) = metrics.add(&event.post_name, &event.origin) {
 error!(“Error writing access metric for {}: {}”, &event.post_name, e);
 } else {
 debug!(“Metric event written for {}”, &event.post_name);
 }
 }
 });

As this function returns nothing, the type is Unit, hence for this lambda I can declare my struct as:

 pub struct MetricHandler {
 receiver_task: JoinHandle<()>,
 //…
 }

And now you can do:

 let receiver_task = tokio::spawn(async move {
 // …
 MetricHandler {
 receiver_task,
 // …
 }